Phase-change memory device with drive circuit

ABSTRACT

A memory device includes an array of phase-change memory cells and a word line. The memory device includes a control circuit, a first pull-up MOSFET and a second pull-up MOSFET connected in series between a first power-supply node set at a first supply voltage and the word line, a first pull-down MOSFET and a second pull-down MOSFET connected in series between the word line and a second power-supply node set at a reference potential, and a biasing MOSFET connected between the word line and a third power-supply node set at a second supply voltage higher than the first supply voltage. The first and second pull-up MOSFETs and the first and second pull-down MOSFETs have breakdown voltages lower than the breakdown voltage of the biasing MOSFET.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Italian Patent Application No.102016000121631, filed on Nov. 30, 2016, which application is herebyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a phase-change memory (PCM) device,which includes a circuit for driving a word line at a high speed.

BACKGROUND

As is known, phase-change memories are new-generation non-volatilememories in which, in order to store information, characteristics ofmaterials having the property of switching between phases with differentelectrical characteristics are exploited. These materials may switchbetween a disorderly/amorphous phase and a crystalline orpolycrystalline orderly phase; different phases are characterized bydifferent values of resistivity and consequently are associated todifferent values of a stored data item. For example, the elements ofGroup VI of the Periodic Table, such as tellurium (Te), selenium (Se),or antimony (Sb), also known as chalcogenides or chalcogenic materials,may be used for manufacturing phase-change memory cells. In particular,an alloy formed by germanium (Ge), antimony (Sb), and tellurium (Te),known as GST (having the chemical composition Ge₂Sb₂Te₅), is currentlywidely used in these memory cells.

Phase changes may be obtained by increasing locally the temperature ofthe cells of chalcogenic material, through resistive electrodes (knownin general as heaters) arranged in contact with corresponding regions ofchalcogenic material.

Access (or selection) devices (for example, MOSFETs) are connected tothe heaters and selectively enable passage of an electric programmingcurrent through them. This electric current, by the Joule effect,generates the temperatures required for phase change.

In particular, when the chalcogenic material is in the amorphous state,and thus has a high resistivity (the so-called RESET state), it isnecessary to apply a current/voltage pulse (or a suitable number ofcurrent/voltage pulses) of a duration and amplitude such as to enablethe chalcogenic material to cool down slowly. Subjected to thistreatment, the chalcogenic material changes its state and switches fromthe high-resistivity state to a low-resistivity state (the so-called SETstate). Conversely, when the chalcogenic material is in the SET state,it is necessary to apply a current/voltage pulse having an appropriateduration and a high amplitude, so as to cause the chalcogenic materialto return into the high-resistivity amorphous RESET state.

During reading, the state of the chalcogenic material is detected byapplying a voltage sufficiently low as not to cause sensible heatingthereof, and by then reading the value of the current that flows in thememory cell through a sense amplifier. Given that the current isproportional to the conductivity of the chalcogenic material, it ispossible to determine in which state the material is set, andconsequently to determine the data item stored in the memory cell.

FIG. 1 shows a PCM device 1, which comprises a memory array 2 formed bya plurality of memory cells 3, arranged in rows, or word lines, andcolumns, or bit lines. Purely by way of example, the memory array 2shown in FIG. 1 has three word lines, designated by WL, and three bitlines, designated by BL, which enable addressing of nine memory cells 3.

Each memory cell 3 is formed by a storage element 4 a and by an accesselement 4 b, which are connected in series between a respective bit lineBL and a terminal at a reference potential (for example, ground).

The storage element 4 a includes an element of phase-change material(for example, a calchogenide, such as GST) and consequently is able tostore data in the form of levels of resistance associated to thedifferent phases assumed by the material itself.

The access element 4 b is formed by an N-channel MOSFET, the drainterminal of which is connected to a first terminal of the storageelement 4 a, the second terminal of which is connected to acorresponding bit line BL. The source terminal of the MOSFET isconnected to ground, whereas the gate terminal is connected to acorresponding word line WL. In this regard, a word line WL is defined bythe set of the gate terminals of the access elements 4 b aligned along asame row; instead, a bit line BL is defined by the set of the secondterminals of the storage elements 4 a aligned along a same column.

In practice, given a memory cell 3, the second terminal of the storageelement 4 a and the gate terminal of the access element 4 b form,respectively, a bit-line terminal and a word-line terminal of the memorycell 3.

The PCM device 1 further comprises a column decoder 8 and a row decoder10, which enable selection of the memory cells 3, on the basis ofaddress signals received at input (designated as a whole by AS). Theaddress signals AS may be generated by a control logic 11, which furthercontrols the column decoder 8 and the row decoder 10 so as to enablereading and writing (also known as programming) of the memory cells 3addressed by the address signals AS. Albeit not shown, the control logic11 supplies to the column decoder 8 and to the row decoder 10 alsocontrol signals in order to control the aforementioned read/writeoperations.

The column decoder 8 and the row decoder 10 enable biasing, and thusselection, of the word lines WL and of the bit lines BL each timeaddressed so as to select the memory cells 3 connected thereto. In thisway, reading and writing of the memory cells 3 are enabled.

In greater detail, the row decoder 10 is designed to select, on thebasis of the address signals AS, a corresponding word line WL. The otherword lines WL are deselected. For this purpose, the row decoder 10comprises a decoding stage 12 and a plurality of driving circuits 14.

The decoding stage 12 receives the address signals AS and controls thedriving circuits 14 on the basis of the address signals AS. Each drivingcircuit 14 then has an input, which is connected to the decoding stage12. Each driving circuit 14 further has an output, which is connected toa corresponding word line WL. In addition, each driving circuit 14 isformed by a corresponding number of MOSFETs (not visible in FIG. 1). Forexample, each driving circuit 14 may be formed by a correspondinginverter circuit.

In practice, the driving circuit 14 biases, and thus controls, the gateterminals of the access elements 4 b connected to the corresponding wordline WL so as to select/deselect the word line WL, on the basis theaddress signals AS.

As regards the column decoder 8, it should be noted that the operationsof programming of the SET and RESET states may be carried out on “words”containing a number N_(b) of bits (N_(b) being an integer higher than orequal to 1), i.e., on a number N_(b) of memory cells 3 connected to asame word line WL. The column decoder 8 is consequently designed toselect simultaneously, on the basis of the address signals AS, a set ofN_(b) bit lines BL, which will also be referred to as “set of bit linesto be programmed”.

In practice, the column decoder 8 co-operates with the row decoder 10 sothat, during the steps of reading or programming of any memory cell 3selected, through the storage element 4 a of this memory cell 3 a readcurrent or a programming current, respectively, flows. For this purpose,the column decoder 8 is configured to provide internally two distinctpaths towards the bit lines BL of the memory array 2 each time selected:a reading path, which during the reading step electrically connects eachbit line BL selected to a sense-amplifier stage 17; and a programmingpath, which during the programming step electrically connects each bitline BL selected to a writing stage 18.

The sense-amplifier stage 17 is configured to compare the read currentthat circulates in the memory cell 3 selected with a reference currentin order to determine the data item stored in the memory cell 3selected. The writing stage 18 is configured to supply the programmingcurrent, which in turn depends upon whether a SET state or a RESET stateis programmed in the memory cell 3 selected.

As regards programming of the memory cells 3, it should be noted that,during the writing operations, it is necessary to supply to the storageelements 4 a current pulses of a high value, both in the case ofprogramming of the SET state and in the case of programming of the RESETstate.

For example, programming of the SET state may be obtained through atrapezial current pulse having an amplitude, for example, comprisedbetween 100 μA and 200 μA, whereas programming of the RESET state may beobtained through a rectangular current pulse having a higher amplitude,for example, between 200 μA and 700 μA.

As regards, instead, the read current, it has values (for example, 30PA) lower than the write current, so as not to damage the stateprogrammed.

This being said, PCM devices afford numerous advantages, amongst which ahigh scalability. However, on account of the high programming currents,the MOSFETs that form the driving circuits 14 must be able to withstandvoltages that are not particularly low (for example, 1.8 V). For thispurpose, these MOSFETs may be manufactured with the so-called 150-nmtechnology; however, the driving circuits 14 thus manufactured arerelatively slow.

In practice, the aforementioned driving circuits 14 are optimized forcarrying out the programming operations, which envisage that selectionof the word lines WL may occur in not particularly short times (forexample, longer than 50 ns) and entail generation of relatively highvoltages. However, in some contexts of application (for example, in theautomotive sector), there is felt the need to be able to select the wordlines WL in a fast way (for example, in times shorter than 3 ns) duringthe reading step. This result appears to be problematical to obtain withjust the use of the aforementioned MOSFETs, without incurring in aconsiderable increase in the area occupation, and thus in the costs.

SUMMARY

Embodiments of the present invention provide a phase-change memorydevice that will solve at least in part the problems of the known art.

An example of phase-change memory device according to the inventionincludes an array of memory cells, which include phase-change material,and a word line. The memory device also includes a driving circuit acontrol circuit, which may be controlled so as to operate in a read modeor in a write mode.

A first pull-up MOSFET and a second pull-up MOSFET have channels of afirst type and are connected in series between a first power-supply nodeconfigured to be set at a first supply voltage and the word line. Thesecond pull-up MOSFET is arranged between the first pull-up MOSFET andthe word line. A first pull-down MOSFET and a second pull-down MOSFEThave channels of a second type and are connected in series between theword line and a second power-supply node configured to be set at areference potential. The second pull-down MOSFET is arranged between thefirst pull-down MOSFET and the word line. A biasing MOSFET is connectedbetween the word line and a third power-supply node and is configured tobe set at a second supply voltage higher than the first supply voltage.

The first and second pull-up MOSFETs and the first and second pull-downMOSFETs have breakdown voltages lower than the breakdown voltage of thebiasing MOSFET. The control circuit is configured to control the firstpull-up MOSFET and the first pull-down MOSFET so that they will operatein a state of switching-on allowed and in a state of switching-onforbidden in a way alternating with respect to one another, as afunction of an input signal indicating whether the word line is to beselected or deselected.

When operating in read mode and when operating in write mode and withthe input signal indicating that the word line is to be deselected, thecontrol circuit is configured to control the biasing MOSFET so as todecouple electrically the word line from the third power-supply node andfor controlling the second pull-up MOSFET and the second pull-downMOSFET so that the first pull-up MOSFET and the first pull-down MOSFETare electrically coupled to the word line. When operating in write modeand with the input signal indicating that the word line is to beselected, the control circuit is configured to control the biasingMOSFET so as to couple electrically the word line to the thirdpower-supply node and for controlling the second pull-up MOSFET and thesecond pull-down MOSFET so that the second pull-up MOSFET is in thestate of switching-on forbidden and that the second pull-down MOSFET isin the state of switching-on allowed.

While this one example has been described, it is understood that theinvention includes other embodiments as disclosed and claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, preferredembodiments thereof will now be described, purely by way of non-limitingexample, with reference to the attached drawings, wherein:

FIG. 1 shows a block diagram of a PCM device;

FIG. 2 shows a block diagram of a portion of a PCM device that includesthe present circuit for driving a word line;

FIG. 3 is a schematic cross-sectional view of a portion of the presentcircuit for driving a word line; and

FIG. 4 is a schematic block diagram of a possible electronic apparatusthat incorporate a PCM device that includes the present circuit fordriving a word line.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present PCM device stems from the fact that the present Applicanthas noted how optimization of a PCM device clashes with the differentrequirements that arise during the writing and reading steps.

In detail, the present Applicant has noted how, in order to speed up thereading step, it is preferable for the MOSFETs that form the drivingcircuits 14 to have a reduced thickness of the gate oxide (for example,to be transistors manufactured with the so-called 28-nm technology).These transistors are also characterized by a reduced area occupation.Instead, in order to prevent damage to the driving circuits 14 duringthe writing step, it is necessary for the MOSFETs that form the drivingcircuits 14 to have a large thickness of the gate oxide. Thesetransistors could thus be manufactured with the 150-nm technology.

This being said, FIG. 2 shows a driving circuit (designated by 34),which is designed to function in a non-volatile PCM device of the typeshown in FIG. 1. Just a part of this PCM device is shown in FIG. 2,where the PCM device is designated by 40. By way of example, the PCMdevice 40 may be the same as the PCM device 1 shown in FIG. 1, but forthe presence of the driving circuit 34. For this reason, hereinafter thecomponents of the PCM device 40 will be designated by the samereferences adopted in FIG. 1 and are not described again.

The driving circuit 34 comprises a logic inverter 42 of a known type,the input terminal of which forms a first input node IN1 of the drivingcircuit 34, which is connected to the decoding stage 12 (not shown inFIG. 2), so that the voltage applied thereon is controlled, in fact, bythe decoding stage 12, on the basis of the address signals AS and in aper se known manner. The output terminal of the logic inverter 42 formsa control node CTRL of the driving circuit 34.

The driving circuit 34 further comprises a logic gate 44 of an AND type.A first input of the logic gate 44 is connected to the first input nodeIN1, whereas a second input of the logic gate 44 forms a second inputnode IN2 of the driving circuit 34. The second input node IN2 of thedriving circuit 34 is connected (in a way not shown) to the controllogic 11.

Albeit not shown in FIG. 2, the logic inverter 42 and the logic gate 44are supplied with a first supply voltage VDD_LV, which is, for example,equal to 1 V.

The driving circuit 34 further comprises a level-shifter circuit 46,which has an input, connected to the output of the logic gate 44, and afirst output and a second output, described hereinafter. Further, albeitnot shown in FIG. 2, the level-shifter circuit 46 is supplied with asecond supply voltage VDD_HV, which is higher than the first supplyvoltage VDD_LV and is, for example, equal to 1.8 V.

The driving circuit 34 further comprises a first MOSFET PM1 and a secondMOSFET NM1, which will be referred to in what follows, respectively, as“first pull-up transistor PM1” and “first pull-down transistor NM1”.Further, the driving circuit 34 comprises a third MOSFET PM2 and afourth MOSFET NM2, which will be referred to in what followsrespectively as “second pull-up transistor PM2” and “second pull-downtransistor NM2.”

The first and second pull-up transistors PM1, PM2 are of the P-channelenhancement type, whereas the first and second pull-down transistorsNM1, NM2 are of the N-channel enhancement type.

The gate terminals of the first pull-up transistor PM1 and of the firstpull-down transistor NM1 are connected to the output terminal of thelogic inverter 42, and thus to the control node CTRL. The sourceterminal of the first pull-up transistor PM1 is set at the first supplyvoltage VDD_LV, whereas the source terminal of the first pull-downtransistor NM1 is set at ground.

The drain terminal of the first pull-up transistor PM1 is connected to afirst conduction terminal of the second pull-up transistor PM2. Thesecond conduction terminal of the second pull-up transistor PM2 isconnected to the word line WL. In general, these terminals will bereferred to as “first and second conduction terminals of the secondpull-up transistor PM2,” without specifying the correspondingsource/drain functions, since, as clarified hereinafter, these functionsvary according to the operating mode of which the PCM device 40.

The gate terminal of the second pull-up transistor PM2 is connected tothe first output of the level-shifter circuit 46.

The drain terminal of the first pull-down transistor NM1 is connected tothe source terminal of the second pull-down transistor NM2, the drainterminal of which is connected to the word line WL. Further, the gateterminal of the second pull-down transistor NM2 is set at a voltageVcasc, which is, for example, 1 V (i.e., it is equal to the first supplyvoltage VDD_LV) and may be generated by an appropriate circuitry (notshown).

Albeit not represented in FIG. 2, the bulk of the second pull-uptransistor PM2 and, optionally, of the first pull-up transistor PM1 areset at the second supply voltage VDD_HV. The bulk of the first andsecond pull-down transistors NM1, NM2 are set at ground.

The driving circuit 34 further comprises a fifth MOSFET PM3, which willbe referred to in what follows as “high-voltage transistor PM3”, forreasons that will be clarified hereinafter.

The high-voltage transistor PM3 is of the P-channel enhancement type.The source terminal of the high-voltage transistor PM3 is set at thesecond supply voltage VDD_HV, whereas the drain terminal is connected tothe word line WL. The gate terminal of the high-voltage transistor PM3is connected to the second output of the level-shifter circuit 46. Thebulk of the high-voltage transistor PM3 is set at the second supplyvoltage VDD_HV.

In greater detail, the first and second pull-up transistors PM1, PM2 andthe first and second pull-down transistors NM1, NM2 are MOSFETs with athin gate oxide, i.e., they are relatively fast transistors, but with arelatively limited capacity for withstanding high voltages. For example,the first and second pull-up transistors PM1, PM2 and the first andsecond pull-down transistors NM1, NM2 may be manufactured with the 28-nmtechnology, in which case they are able to withstand gate-to-source,gate-to-drain, and drain-to-source voltages not higher than 1.1 V.Instead, the high-voltage transistor PM3 is a transistor with a gateoxide having a thickness greater than the thickness of the gate oxidesthat form the first and second pull-up transistors PM1, PM2 and thefirst and second pull-down transistors NM1, NM2. Consequently, thehigh-voltage transistor PM3 is relatively slow, but has the capacity forwithstanding high voltages. For example, the high-voltage transistor PM3may be manufactured with the 150-nm technology, in which case it is inable to withstand up to 1.8 V.

Without any loss of generality, the first and second pull-up transistorsPM1, PM2, the first and second pull-down transistors NM1, NM2, and thehigh-voltage transistor PM3 may be integrated in a single die, adoptingthe so-called fully-depleted silicon-on-insulator (FDSOI) technology.FIG. 3 is a qualitative representation of a portion of the die(designated by 50) and, purely by way of example, of the first pull-uptransistor PM1.

In detail, the die 50 comprises a substrate 51 of semiconductormaterial, which is delimited at the top by a top surface S_(sup) and hasa doping of a P type, and a well 49, which has a doping of an N type andextends in the substrate 51, starting from the top surface S_(sup). Atrench 52 extends in the well 49, starting from the top surface S_(sup).This trench 52 has a shape that is, for example, annular in top planview and is filled by an annular insulation region 53, which in turndelimits laterally an internal region 54 of the well 49.

The first pull-up transistor PM1 further comprises a buried region 55,of dielectric material, which extends in the internal region 54 at adistance from the top surface S_(sup), until it comes into contact withthe annular insulation region 53. The buried region 55 and the annularinsulation region 53 delimit at the bottom and laterally an activeregion 56 of semiconductor material 56, of an N type.

The first pull-up transistor PM1 further comprises a drain region 57 anda source region 58, which are of a P type, are laterally staggered withrespect to one another, and extend in the active region 56 starting fromthe top surface S_(sup), until they come into contact with the buriedregion 55. The portion of active region 56 not occupied by the drainregion 57 and by the source region 58 forms a body region 59.

The first pull-up transistor PM1 further comprises a gate-oxide region 6o, which extends over the top surface S_(sup), overlying the body region59, in direct contact therewith, as well as portions of the drain region57 and of the source region 58. Present on the gate-oxide region 60 is aconductive region 61, which is laterally surrounded by a top dielectricregion 62.

Albeit not shown, also the second pull-up transistor PM2 and the firstand second pull-down transistors NM1, NM2, and the high-voltagetransistor PM3 are formed in the die 50. The second pull-up transistorPM2 is, for example, the same as the first pull-up transistor PM1.Further, the first and second pull-down transistors NM1, NM2 are thesame as the first pull-up transistor PM1, but for the absence of thewell 49 (or of a different type of conductivity of the well 49) and forthe fact that the types of doping of the body region, of the drainregion, and of the source region are reversed. As regards, instead, thehigh-voltage transistor PM3, it differs from the first and secondpull-up transistors PM1, PM2 as regards the dimensions (in particular,the thickness) of the gate-oxide region, as mentioned previously.

Advantages deriving from adoption of the FDSOI technology are describedhereinafter.

Irrespective of the detail regarding a possible implementation of theMOSFETs, the decoding stage 12 generates on the first input node IN1,and thus on the first input of the logic gate 44, a signal s_(in),indicating whether the word line WL is to be selected or deselected. Onthe control node CTRL a signal s_(ctrl1) is, instead, present, which isequal to the logic negation of the signal s_(in) and controls the firstpull-up transistor PM1 and the first pull-down transistor NM1.

On the second input node IN2 a signal s_(MODIFY) is present, describedhereinafter and generated by the control logic 11; on the output of thelogic gate 44 a signal s_(AND) is, instead, present.

As previously mentioned, the logic inverter 42 and the logic gate 44operate in the domain of the first supply voltage VDD_LV. Consequently,each of the signals s_(in), s_(ctrl1), s_(MODIFY) and s_(AND) assumes avalue, for example, equal to 1 V, when it indicates the logic value ‘1’,whereas it is approximately zero when it indicates the logic value ‘0’.

This being said, the level-shifter circuit 46 generates, instead, asignal s_(ctrl2) and a signal ns_(ctrl2), respectively on its own firstand second outputs. Consequently, the signal s_(ctrl2) controls thesecond pull-up transistor PM2, whereas the signal ns_(ctrl2) controlsthe high-voltage transistor PM3.

In greater detail, the signal s_(ctrl2) represents a version of thesignal s_(AND) shifted in the domain of the second supply voltageVDD_HV; i.e., it represents the same logic values as those of the signals_(AND), but the logic value ‘1’ is associated to a voltageapproximately equal to 1.8 V. The signal ns_(ctrl2) is the logicnegation of the signal s_(ctrl2), and also this belongs to the domain ofthe second supply voltage VDD_HV.

All this being said, assuming that the address signals AS are such thatthe driving circuit 34 has to select its own word line WL, i.e.,assuming that the signal s_(in) is equal to ‘1’, and further assumingthat it is necessary to carry out a reading operation, i.e., assumingthat the signal s_(MODIFY) is equal to ‘0’, the following occurs.

The signals s_(ctrl1), s_(AND), and s_(ctrl2) are equal to ‘0’, whereasthe signal ns_(ctrl2) is equal to ‘1’; thus, it has a voltage equal tothe second supply voltage VDD_HV. Consequently, the gate terminals ofthe first pull-down transistor NM1 and of the first and second pull-uptransistors PM1, PM2 are set at a zero voltage, whereas the gateterminal of the high-voltage transistor PM3 is set at 1.8 V.Consequently, the high-voltage transistor PM3 and the first pull-downtransistor NM1 are de-activated, whereas the first and second pull-uptransistors PM1, PM2 and the second pull-down transistor NM2 are active.In this connection, in the present description a MOSFET the gateterminal of which is at a voltage such that passage of current throughthe MOSFET itself is not prevented a priori (i.e., irrespective of thestate of the other MOSFETs) will be referred to as “active MOSFET”.

Equivalently, an active MOSFET is in a state of switching-on allowed,which does not necessarily imply a passage of current through thetransistor, since the effective onset of this passage depends, in a perse known manner, upon further conditions on the gate voltage (inparticular, upon the voltages on the conduction terminals). As describedhereinafter, and without this implying any loss of generality, the stateof switching-on allowed of a MOSFET implies that the corresponding gateterminal is controlled with a respective gate signal (in particular, thesignals s_(ctrl1), Vcasc, s_(ctrl2) and ns_(ctrl2)), which:

in the case of an N channel, is equal to the first supply voltage VDD_LV(or Vcasc, in the case of the second pull-down transistor NM2), whichrepresents a sort of logic value ‘1’ of this gate signal; and

in the case of a P channel, is zero, i.e., it assumes a sort of logicvalue ‘0’.

Likewise, in the present description a MOSFET the gate terminal of whichis at a voltage such that the passage of current is prevented a prioriwill be referred to as “de-activated MOSFET”. Equivalently, ade-activated MOSFET is in a state of switching-on forbidden; without anyloss of generality, the state of switching-on forbidden of a MOSFETimplies that:

in the case of an N channel, the corresponding gate signal is zero;

in the case of a P channel, the corresponding gate signal is equal tothe second supply voltage VDD_HV (which represents a logic value ‘1’, inthe respective voltage domain), if the MOSFET is either the secondpull-up transistor PM2 or the high-voltage transistor PM3, or else isequal to the first supply voltage VDD_LV, if the MOSFET is the firstpull-up transistor PM1.

In practice, since the second pull-up transistor PM2 and the secondpull-down transistor NM2 are active, the circuit formed by the first andsecond pull-up transistors PM1, PM2 and by the first and secondpull-down transistors NM1, NM2 functions as an inverter controlled bythe signal s_(ctrl1), which in turn controls the word line WL. Further,the word line WL is biased at approximately 1 V, through the first andsecond pull-up transistors PM1, PM2; consequently, the word line WL isdecoupled from the second supply voltage VDD_HV.

In the case where, instead, a reading operation is carried out in whichthe word line WL is to be deselected, i.e., assuming that the signals_(in) is equal to ‘0’ and that the signal s_(MODIFY) is also equal to‘0’, the signal s_(ctrl1) becomes equal to ‘1’, whereas the othersignals do not change. Consequently, the high-voltage transistor PM3remains de-activated, whereas the second pull-up transistor PM2 and thesecond pull-down transistor NM2 remain active. The first pull-uptransistor PM1 is de-activated, whereas the first pull-down transistorNM1 is active. Consequently, the circuit formed by the first and secondpull-up transistors PM1, PM2 and by the first and second pull-downtransistors NM1, NM2 once again functions as an inverter controlled bythe signal s_(ctrl1), which deselects the word line WL, connecting it toground through the first and second pull-down transistors NM1, NM2.

In practice, during a reading operation, the word line WL isselected/deselected only through fast MOSFETs. Further, in the casewhere the word line WL is selected, it is biased at 1 V. In addition,since the high-voltage transistor PM3 remains de-activated, theremaining transistors are not subject to high voltages.

Assuming, instead, that a programming operation is carried out on theword line WL, i.e., assuming that the signal s_(MODIFY) is equal to thelogic value ‘1’, and assuming further that the word line WL is to beselected (signal s_(in) equal to ‘1’), the following occurs.

The signal s_(AND) assumes a logic value ‘1’, whereas the signals_(ctrl1) has a logic value ‘0’. Further, the signal s_(ctrl2) assumesthe logic value ‘1’, whereas the signal ns_(ctrl2) assumes the logicvalue ‘0’. The gate terminals of the first pull-down transistor NM1, ofthe first pull-up transistor PM1, and of the high-voltage transistor PM3are set at ground. The gate terminal of the second pull-up transistorPM2 is set at 1.8 V. Consequently, the high-voltage transistor PM3 isactive, as likewise the first pull-up transistor PM1 and the secondpull-down transistor NM2, whereas the second pull-up transistor PM2 andthe first pull-down transistor NM1 are de-activated.

In practice, during the programming operation, the word line WL isselected through the high-voltage transistor PM3, which biases it at 1.8V.

Notwithstanding the fact that the word line WL is biased at 1.8 V, noneof the first and second pull-up transistors PM1, PM2 and of the firstand second pull-down transistors NM1, NM2 is subjected to voltages thatrisk damaging it, since each of them is operating with drain-to-source,gate-to-source, and gate-to-drain voltages not higher than the firstsupply voltage VDD_LV, as explained in what follows, where theaforementioned three voltages will be denoted, respectively, by V_(ds),V_(gs) and V_(gd).

In detail, as regards the first pull-up transistor PM1, we have (inmodulus): V_(gs)=VDD_LV, V_(gd)=VDD_LV, and V_(ds)=0.

As regards the second pull-up transistor PM2, we have (in modulus):V_(gs)=0, V_(gd)=VDD_HV−VDD_LV, and V_(ds)=VDD_HV−VDD_LV.

As regards the first pull-down transistor NM1, we have (in modulus):V_(gs)=0, V_(gd)=Vcasc−V_(th) _(_) _(NM2), and V_(ds)=Vcasc−V_(th) _(_)_(NM2), where V_(th) _(_) _(NM2) is the threshold voltage of the secondpull-down transistor NM2 and is, for example, equal to 0.4V.

As regards the second pull-down transistor NM2, we have (in modulus):V_(gs)=V_(th) _(_) _(NM2) (the second pull-down transistor NM2, albeitactive, is not traversed by current, on account of the insufficientV_(gs)), V_(gd)=VDD_HV−Vcasc, and V_(ds)=VDD_HV−(Vcasc−V_(th) _(_)_(NM2)) (the latter voltage may occasionally exceed the first supplyvoltage VDD_LV, because current technology enables voltages higher thanVDD_LV to be withstood for short periods).

Further, in the case where transistors of the FDSOI type are adopted,the advantages outlined in what follows are achieved.

With reference, for example, to the second pull-up transistor PM2, eventhough its second conduction terminal is set at the second supplyvoltage VDD_HV, its bulk may in any case be biased at the first supplyvoltage VDD_LV, instead of at the second supply voltage VDD_HV, withoutthis entailing entry into forward biasing of the junction formed by theterminal set at VDD_HV and by the bulk, thanks to the insulation of thebulk. This bestows greater speed on the second pull-up transistor PM2,given the same area occupied, as compared to the case (mentionedpreviously) where the bulk is set conservatively at the second supplyvoltage VDD_HV. In this connection, in the case where the bulk of thesecond pull-up transistor PM2 is not insulated, an alternative that willmake it possible not to penalize the speed or occupation of area wouldconsist in pre-arranging a circuitry (not shown) designed to set thebulk of the second pull-up transistor PM2 alternatively at the firstsupply voltage VDD_LV or at the second supply voltage VDD_HV, accordingto whether a reading operation or a programming operation is carriedout. Similar considerations apply in regard to the other transistors,and in particular in regard to the first pull-up transistor PM1 and tothe high-voltage transistor PM3.

Finally, assuming that a programming operation is carried out in whichthe word line WL is deselected, i.e., assuming that the signals_(MODIFY) is equal to the logic value ‘1’ and that the signal s_(in) isequal to ‘0’, the following occurs.

The signal s_(AND) assumes a logic value ‘0’, whereas the signals_(ctrl1) has a logic value ‘1’. Further, the signal s_(ctrl2) assumesthe logic value ‘0’, whereas the signal ns_(ctrl2) assumes the logicvalue ‘1’. The gate terminals of the first pull-down transistor NM1 andof the first pull-up transistor PM1 are set at the first supply voltageVDD_LV. The gate terminal of the second pull-up transistor PM2 is set atground, whereas the gate terminal of the high-voltage transistor PM3 isset at the second supply voltage VDD_HV. Consequently, the high-voltagetransistor PM3 is de-activated, as also the first pull-up transistorPM1, whereas the second pull-up transistor PM2 and the first and secondpull-down transistors NM1, NM2 are active. In this way, the word line WLis set at ground through the first and second pull-down transistors NM1,NM2. Since in this operation the high-voltage transistor PM3 remainsde-activated, the second supply voltage VDD_HV remains decoupled fromthe word line WL, and thus the driving circuit 34 does not risk beingexposed to overvoltages.

In general, irrespective of the considerations regarding the bulk, thesecond pull-up transistor PM2 and the second pull-down transistor NM2function as cascode transistors, since they are active and have noeffect in the reading step, whereas they enable protection of the firstpull-up transistor PM1 and the first pull-down transistor NM1 during theprogramming step (in particular, in the case of selection of the wordline). In fact, during the programming step with selection of the wordline WL, the presence of the second pull-down transistor NM2 causes areduction of the voltages V_(gd) and V_(ds), below the first supplyvoltage VDD_LV, even though the word line WL is set at the second supplyvoltage VDD_HV. Further, the presence of the second pull-up transistorPM2 makes it possible to maintain the first pull-up transistor PM1active, without subjecting it to voltages higher than the first supplyvoltage VDD_LV. In turn, the second pull-up transistor PM2 is protectedthanks to the control based upon the signal s_(ctrl2), which belongs tothe voltage domain of the second supply voltage VDD_HV.

In addition, when the first pull-down transistor NM1 is switched on, thevoltage of its drain terminal is approximately equal to VDD_HV/2, on thehypothesis where the first and second pull-down transistors NM1, NM2 arethe same as one another, at the start of a step of discharge of the wordline WL.

The advantages of the solution discussed are evident from the foregoingdescription.

In particular, thanks to the use of fast MOSFETs, the present drivingcircuit makes it possible to carry out reading operations in a very fastway (in times shorter than 3 ns), albeit enabling correct selection ofthe word line WL during the programming step, without exposing theMOSFETs to overvoltages. Further, the present driving circuit ischaracterized by a reduced occupation of area.

Finally, it is clear that modifications and variations may be made towhat is described and illustrated herein, without thereby departing fromthe scope of the present invention, as defined in the annexed claims.

In particular, the present driving circuit is independent of thecharacteristics of the memory cells.

As regards the voltage Vcasc, it is higher than the voltage V_(th) _(_)_(NM2) and may have a value different from the one described. Ingeneral, in order to protect the first and second pull-down transistorsNM1, NM2, we have, in addition to Vcasc≤VDD_LV, |Vcasc−VDD_HV|≤VDD_LVand VDD_HV−(Vcasc−V_(th) _(_) _(NM2))<VDD_LV. More precisely, respect ofthe above inequalities ideally guarantees a protection for an indefinitetime. However, the aforementioned inequalities may not be respected forlimited periods of time, as a function of the service life realisticallyexpected for the PCM device 40.

Further, it is possible for the signals s_(ctrl1), s_(ctrl2) andns_(ctrl2) to be generated in a way different from what has beendescribed. Consequently, the logic inverter 42 and the logic gate 44 maybe replaced by a different circuitry.

As regards the signal s_(ctrl2), it may assume, at its own logic value‘1’, a value different from the second supply voltage VDD_HV, providedthat it is sufficient to de-activate the second pull-up transistor PM2during the operations of programming of the word line WL. Consequently,it is sufficient for the signal s_(ctrl2) to assume, at its own logicvalue ‘1’, a voltage higher than or equal to VDD_HV−|V_(th) _(_)_(PM2)|, where V_(th) _(_) _(PM2) is the threshold voltage of the secondpull-up transistor PM2. Similar considerations apply for the signalns_(ctrl2) and to switching-off of the high-voltage MOSFET PM3.

The fast transistors (i.e., the first and second pull-down transistorsand the first and second pull-up transistors) may have, for example,gate oxides with thicknesses different from one another; more ingeneral, the fast transistors may have breakdown voltages different fromone another, in any case lower than the breakdown voltage of thehigh-voltage transistor. It is further possible for the high-voltagetransistor to have a gate oxide with the same thickness as the gateoxides of the fast transistors, albeit continuing to have a breakdownvoltage higher than the breakdown voltages of the fast transistors.

As regards, instead, the MOSFETs that form the access elements 4 b,their characteristics are irrelevant for the purposes of the presentinvention. For example, they may be manufactured with the sametechnology used for manufacturing the driving circuit 34.

In addition, it is evident that the PCM device 40 may find use innumerous contexts, such as applications with high safety requirementsthat use smart cards with contact interface (such as pay-TV systems) andhave to satisfy stringent consumption specifications, or elseapplications that use contactless smart cards (RFIDs, NFCs, bank creditcards, etc.), where the memory module has a current budget obtainedlimited by an energy-harvesting system.

In this connection, FIG. 4 illustrates a portion of an electronicapparatus 70, which may, for example, be a PDA (personal digitalassistant), a portable or fixed computer (possibly with capacity ofwireless data transfer), a cell phone, a digital audio player, aphotographic camera or a camcorder, or further devices that are able toprocess, store, transmit, and receive information.

In detail, the electronic apparatus 70 comprises: a controller 71 (forexample, provided with a microprocessor, a DSP, or a microcontroller);an input/output device 72 (for example, provided with a keypad and adisplay), for input and display of the data; the PCM device 40 providedwith the array 2 of memory cells 3 of the phase-change type describedpreviously; a wireless interface 74, for example, an antenna, fortransmitting and receiving data through a radiofrequency wirelesscommunication network; and a RAM 75. All the components of theelectronic apparatus 70 are coupled through a bus 76. It is possible touse a battery 77 as power-supply source in the electronic apparatus 70,which may further be equipped with a photographic camera or video cameraor camcorder 78. In addition, the controller 71 may control the PCMdevice 40, for example, co-operating with the control logic 11.

What is claimed is:
 1. A memory device comprising: an array of memorycells, each memory cell including a phase-change material; a word linecoupled to memory cells of the array; a control circuit, which may becontrolled so as to operate in a read mode or in a write mode; a firstpull-up MOSFET and a second pull-up MOSFET having channels of a firsttype, the first and second pull-up MOSFETs being connected in seriesbetween a first power-supply node configured to be set at a first supplyvoltage and the word line, the second pull-up MOSFET being arrangedbetween the first pull-up MOSFET and the word line; a first pull-downMOSFET and a second pull-down MOSFET having channels of a second type,the first and second pull-down MOSFETs being connected in series betweenthe word line and a second power-supply node configured to be set at areference potential, the second pull-down MOSFET being arranged betweenthe first pull-down MOSFET and the word line; and a biasing MOSFETcoupled between the word line and a third power-supply node configuredto be set at a second supply voltage that is higher than the firstsupply voltage, wherein the first and second pull-up MOSFETs and thefirst and second pull-down MOSFETs have breakdown voltages lower than abreakdown voltage of the biasing MOSFET.
 2. The memory device accordingto claim 1, wherein each of the first and second pull-up MOSFETs, thefirst and second pull-down MOSFETs, and the biasing MOSFET is of afully-depleted silicon-on-insulator (FDSOI) transistor.
 3. The memorydevice according to claim 1, wherein the first and second pull-downMOSFETs and the first and second pull-up MOSFETs have respectivegate-dielectric regions with the same first thickness, and wherein thebiasing MOSFET has a gate-dielectric region with a thickness greaterthan the first thickness.
 4. The memory device according to claim 1,wherein each of the first and second pull-up MOSFETs, the first andsecond pull-down MOSFETs, and the biasing MOSFET is an enhancement typeMOSFET.
 5. The memory device according to claim 4, wherein the first andsecond pull-up MOSFETs and the biasing MOSFET have channels with Pconductivity, and wherein the first and second pull-down MOSFETs havechannels with N conductivity.
 6. A memory device comprising: an array ofmemory cells, each memory cell including a phase-change material; a wordline coupled to memory cells of the array; a control circuit, which maybe controlled so as to operate in a read mode or in a write mode; afirst pull-up MOSFET and a second pull-up MOSFET having channels of afirst type, the first and second pull-up MOSFETs being connected inseries between a first power-supply node configured to be set at a firstsupply voltage and the word line, the second pull-up MOSFET beingarranged between the first pull-up MOSFET and the word line; a firstpull-down MOSFET and a second pull-down MOSFET having channels of asecond type, the first and second pull-down MOSFETs being connected inseries between the word line and a second power-supply node configuredto be set at a reference potential, the second pull-down MOSFET beingarranged between the first pull-down MOSFET and the word line; and abiasing MOSFET coupled between the word line and a third power-supplynode configured to be set at a second supply voltage that is higher thanthe first supply voltage; wherein the first and second pull-up MOSFETsand the first and second pull-down MOSFETs have breakdown voltages lowerthan a breakdown voltage of the biasing MOSFET; wherein the controlcircuit is configured to control the first pull-up MOSFET and the firstpull-down MOSFET so that they will operate in a state of switching-onallowed and in a state of switching-on forbidden in a way alternatingwith respect to one another, as a function of an input signal thatindicates whether the word line is to be selected or deselected; andwherein, i) when operating in read mode and ii) when operating in writemode with the input signal indicating that the word line is to bedeselected, the control circuit is configured to control the biasingMOSFET so as to electrically decouple the word line from the thirdpower-supply node and to control the second pull-up MOSFET and thesecond pull-down MOSFET so that the first pull-up MOSFET and the firstpull-down MOSFET are electrically coupled to the word line; and wherein,when operating in write mode and with the input signal indicating thatthe word line is to be selected, the control circuit is configured tocontrol the biasing MOSFET so as to couple electrically the word line tothe third power-supply node and to control the second pull-up MOSFET andthe second pull-down MOSFET so that the second pull-up MOSFET is in thestate of switching-on forbidden and that the second pull-down MOSFET isin the state of switching-on allowed.
 7. The memory device according toclaim 6, wherein the control circuit is configured to set a gateterminal of the second pull-down MOSFET to a cascode voltage, which ishigher than a threshold voltage of the second pull-down MOSFET; andwherein, when operating in write mode and with the input signalindicating that the word line is to be selected, the control circuit isfurther configured to set a gate terminal of the second pull-up MOSFETat a voltage greater than or equal to the difference between the secondsupply voltage and the modulus of the threshold voltage of the secondpull-up MOSFET.
 8. The memory device according to claim 7, wherein thecontrol circuit is configured to generate a first control signal on gateterminals of the first pull-up MOSFET and of the first pull-down MOSFET,and to generate a second control signal, on the gate terminal of thesecond pull-up MOSFET, wherein the first control signal ranges betweenthe reference potential and the first supply voltage and wherein thesecond control signal ranges between the reference potential and thesecond supply voltage.
 9. The memory device according to claim 8,wherein the control circuit is further configured to generate, on a gateterminal of the biasing MOSFET, a third control signal, which rangesbetween the reference potential and the second supply voltage and is thelogic negation of the second control signal.
 10. The memory deviceaccording to claim 6, wherein each of the first and second pull-upMOSFETs, the first and second pull-down MOSFETs, and the biasing MOSFETis of a fully-depleted silicon-on-insulator (FDSOI) transistor.
 11. Thememory device according to claim 6, wherein the first and secondpull-down MOSFETs and the first and second pull-up MOSFETs haverespective gate-dielectric regions with the same first thickness, andwherein the biasing MOSFET has a gate-dielectric region with a thicknessgreater than the first thickness.
 12. The memory device according toclaim 6, wherein the biasing MOSFET has a channel of the first type. 13.The memory device according to claim 6, wherein each of the first andsecond pull-up MOSFETs, the first and second pull-down MOSFETs, and thebiasing MOSFET is an enhancement type MOSFET.
 14. The memory deviceaccording to claim 13, wherein the first and second pull-up MOSFETs andthe biasing MOSFET have channels with P conductivity, and wherein thefirst and second pull-down MOSFETs have channels with N conductivity.15. The memory device according to claim 6, wherein each memory cellcomprises a storage element and an access element, which areelectrically coupled together; wherein each storage element is formed bythe phase-change material; and wherein each access element is formed bya corresponding MOSFET.
 16. An electronic apparatus comprising: thememory device according to claim 6; a controller; and a bus electricallycoupling the controller and the memory device.
 17. A method for biasinga word line of a memory device that comprises an array of phase-changememory cells, wherein the memory device further comprises: a firstpull-up MOSFET and a second pull-up MOSFET having channels of a firsttype, the first and second pull-up MOSFETs being connected in seriesbetween a first power-supply node configured to be set at a first supplyvoltage and the word line and the second pull-up MOSFET being arrangedbetween the first pull-up MOSFET and the word line; a first pull-downMOSFET and a second pull-down MOSFET having channels of a second type,the first and second pull-down MOSFETs being connected in series betweenthe word line and a second power-supply node configured to be set at areference potential and the second pull-down MOSFET being arrangedbetween the first pull-down MOSFET and the word line; and a biasingMOSFET connected between the word line and a third power-supply nodeconfigured to be set at a second supply voltage that is higher than thefirst supply voltage, the first and second pull-up MOSFETs and the firstand second pull-down MOSFETs having breakdown voltages lower than thebreakdown voltage of the biasing MOSFET; the method comprising:controlling the first pull-up MOSFET and the first pull-down MOSFET byan input signal that indicates whether the word line is to be selectedor deselected, so that they will operate in a state of switching-onallowed and in a state of switching-on forbidden in a way alternatingwith respect to one another, as a function of the input signal; during areading step, and during a writing step with the input signal indicatingthat the word line is to be deselected, controlling the biasing MOSFETso as to decouple electrically the word line from the third power-supplynode, and controlling the second pull-up MOSFET and the second pull-downMOSFET so that the first pull-up MOSFET and the first pull-down MOSFETare electrically coupled to the word line; and during a writing stepwith the input signal indicating that the word line is to be selected,controlling the biasing MOSFET so as to couple electrically the wordline to the third power-supply node, and controlling the second pull-upMOSFET and the second pull-down MOSFET so that the second pull-up MOSFETis in the state of switching-on forbidden and the second pull-downMOSFET is in the state of switching-on allowed.
 18. The method ofbiasing according to claim 17, further comprising: setting a gateterminal of the second pull-down MOSFET at a cascode voltage, which ishigher than a threshold voltage of the second pull-down MOSFET; andduring the writing step with the input signal indicating that the wordline is to be selected, setting the gate terminal of the second pull-upMOSFET at a voltage higher than or equal to the difference between thesecond supply voltage and the modulus of a threshold voltage of thesecond pull-up MOSFET.
 19. The method of biasing according to claim 18,further comprising: generating a first control signal on gate terminalsof the first pull-up MOSFET and of the first pull-down MOSFET, whereinthe first control signal ranges between the reference potential and thefirst supply voltage; and generating a second control signal on the gateterminal of the second pull-up MOSFET, wherein the second control signalranges between the reference potential and the second supply voltage.20. The method of biasing according to claim 19, further comprisinggenerating, on a gate terminal of the biasing MOSFET, a third controlsignal, which ranges between the reference potential and the secondsupply voltage and is the logic negation of the second control signal.